Assuming you have already installed mono like you said, then you should be able to install IronAhk, by going to the ironAhk site and downloading your binaries, this comes as a zipped folder, open that and run setup.sh from the terminal.
If you do ./setup.sh it will tell you where it's default install directory will be, on mac /usr
run sudo ./setup.sh install from the terminal and it will install it.
Go into the /usr/bin folder and run IronAhk and it should say
No source path specified.
Give it a source path, the example.ahk provided in the download
This may give you an error message:
Could not execute: An exception was thrown by the type initializer for System.Windows.Forms.WindowsFormsSynchronizationContext
To fix this you will need to install Xquartz:
After which you log out, log in, and run the example above which should give you a little hello world msgBox.